The present invention relates to a seat assembly for a vehicle and, more particularly, to a vehicle seat back assembly which protects a neck portion of a seat occupant in the vehicle such as a cervical vertebrae of the seat occupant during so-called a rear-end collision caused by other vehicle.
In the past, it has heretofore been proposed to provide a vehicle seat back assembly as disclosed in Japanese Patent Application Laid-Open Publication No. 10-119619 wherein the seat back assembly includes a lateral frame member composed of side frames having upper distal ends formed with forwardly extending pivot hinges, and a pressure receiving member responsive to the rear-end collision.
With such a structure, when the vehicles encounters the rear-end collision, the seat back assembly is initially exerted with an impact load (hereinafter referred to as a primary collision) applied in a forward direction of the vehicle and is subsequently subject to a reactive load (hereinafter referred to as a secondary collision) applied in a rearward direction owing to a body per se of a seat occupant. Thus, the pressure receiving member is exerted with the reactive load. When this occurs, the pressure receiving member is moved rearward, causing the lateral frame member to rotate in the forward direction about the centers of the pivot hinges such that the head rest supported with the lateral frame member is moved toward the head of the seat occupant to allow the head rest to move in a direction in close proximity to the head of the seat occupant.
Accordingly, even when the seat back assembly is flexed rearward responsive to the reactive load of the seat occupant during the rear-end collision of the vehicle, the pressure responsive member is responsive to the reactive load to instantaneously cause the head rest to move closer to the head of the seat occupant in accordance with the principle of leverage and, therefore, the head of the seat occupant is reliably retained with the head rest to thereby protect the cervical vertebrae of the seat occupant.
With such a structure, however, since the lateral frame member is constructed of three pieces of component parts including an upper side member, a rear side member and a lower side member, which are welded together by a welding technique, the respective component parts undergo a distortion due to the welding technique, with a resultant unbalance caused in operating forces of a stay of the head rest which are supported with the upper and lower side members. Thus, it is required to eliminate such an unbalance in the operating forces of the stays of the head rest.
The present invention has been made with a view of the aforementioned studies and has an object to provide a vehicle seat back assembly which is able to avoid a distortion to be caused in a head rest support member that supports a head rest.
It is another object of the present invention to provide a vehicle seat back assembly which is simple in construction, highly reliable in operation and low in manufacturing cost.
According to one aspect of the present invention, there is provided a vehicle seat back assembly comprising: a seat back frame including, at both sides thereof, at least side frames; a head rest support member supported with upper distal ends of the side frames to be rotatable; a pressure receiving member supported with the head rest support member and remaining in a position to receive an impact load caused by a rear-end collision of a vehicle; a head rest holder supported with the head rest support member; and a head rest supported with the head rest holder to be moveable in upper and lower directions. The head rest support member has an upper wall segment, a rear facing wall segment bent from the upper wall segment and extending downward, and a support section supporting the head rest holder at the upper wall segment and a portion extending from the rear facing wall segment.